/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineGroupsInterface.h
Ask the server if the given group name currently exists. If it completes successfully, you can call GetCachedGroupNameExist to get the result. Even if the result is currently cached, this function will contact the server (ostensibly to refresh the cache).
void QueryGroupNameExist ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & ContextUserId, const [FString](API\Runtime\Core\Containers\FString) & GroupName, const [FOnGroupsRequestCompleted](API\Plugins\OnlineSubsystem\Interfaces\FOnGroupsRequestCompleted) & OnCompleted )